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  • OULU 1998

    MULTIUSER DEMODULATION FOR DS-CDMA SYSTEMS IN FADING CHANNELS

    MARKKUJUNTTI

    Department of Electrical Engineering

  • MULTIUSER DEMODULATION FOR DS-CDMA SYSTEMS IN FADING CHANNELS

    MARKKU JUNTTI

    Academic Dissertation to be presented with the assent of The Faculty of Technology, University of Oulu, for public discussion in Raahensali (Auditorium L 10), Linnanmaa, on October 17th, 1997, at 12 noon.

    OULUN YLIOP ISTO, OULU 1998

  • Copyright 1998Oulu University Library, 1998

    Manuscript received 16 Sebtember 1997Accepted 18 Sebtember 1997

    Communicated by Professor Timo LaaksoProfessor Sergio Verd

    ALSO AVAILABLE IN PRINTED FORMAT

    ISBN 951-42-4755-8(URL: http://herkules.oulu.fi/isbn9514247558/)

    ISBN 951-42-4632-2ISSN 0355-3213 (URL: http://herkules.oulu.fi/issn03553213/)

    OULU UNIVERSITY LIBRARYOULU 1998

  • Dedicated to my family.

  • Juntti, Markku, Multiuser demodulation for DS-CDMA systems in fad-ing channelsDepartment of Electrical Engineering, University of Oulu, FIN-90570 Oulu, Fin-landActa Univ. Oul. C 106, 1997Oulu, Finland

    (Manuscript received 16 September, 1997)

    Abstract

    Multiuser demodulation algorithms for centralized receivers of asynchronousdirect-sequence (DS) spread-spectrum code-division multiple-access (CDMA) systems infrequency-selective fading channels are studied. Both DS-CDMA systems with short (onesymbol interval) and long (several symbol intervals) spreading sequences are considered.

    Linear multiuser receivers process ideally the complete received data block. The ap-proximation of ideal infinite memory-length (IIR) linear multiuser detectors by finitememory-length (FIR) detectors is studied. It is shown that the FIR detectors can bemade near-far resistant under a given ratio between maximum and minimum receivedpower of users by selecting an appropriate memory-length. Numerical examples demon-strate the fact that moderate memory-lengths of the FIR detectors are sufficient to achievethe performance of the ideal IIR detectors even under severe near-far conditions.

    Multiuser demodulation in relatively fast fading channels is analyzed. The optimalmaximum likelihood sequence detection receiver and suboptimal receivers are considered.The parallel interference cancellation (PIC) receiver is demonstrated to achieve betterperformance in known channels than the decorrelating receiver, but it is observed to bemore sensitive to channel coefficient estimation errors than the decorrelator. At highchannel loads the PIC receiver suffers from bit error rate (BER) saturation, whereas thedecorrelating receiver does not. Choice of channel estimation filters is shown to be crucialif low BER is required. Data-aided channel estimation is shown to be more robust thandecision-directed channel estimation, which may suffer from BER saturation caused byhang-ups at high signal-to-noise ratios.

    Multiuser receivers for dynamic CDMA systems are studied. Algorithms for ideal lin-ear detector computation are derived and their complexity is analyzed. The complexity ofthe linear detector computation is a cubic function of KL, where K and L are the num-ber of users and multipath components, respectively. Iterative steepest descent, conjugategradient, and preconditioned conjugate gradient algorithms are proposed to reduce thecomplexity. The computational requirements for one iteration are a quadratic function ofKL. The iterative detectors are also shown to be applicable for parallel implementation.Simulation results demonstrate that a moderate number of iterations yields the perfor-mance of the corresponding ideal linear detectors. A quantitative analysis shows that thePIC receivers are significantly simpler to implement than the linear receivers and onlymoderately more complex than the conventional matched filter bank receiver.

    Keywords: channel estimation, interference cancellation, decorrelation, iterativedetection

  • Preface

    Research for this thesis has been carried out mostly in the spread-spectrum researchgroup in the Telecommunication Laboratory, Department of Electrical Engineering,University of Oulu, Oulu, Finland. The work towards the thesis was initialized inAugust 1993 after I had completed the Masters thesis in May 1993. The first yearof the research included mostly literature study. From September 1994 to October1995 I was a visiting scholar at Department of Electrical and Computer Engi-neering, Rice University, Houston, Texas, USA in Professor Behnaam Aazhangsresearch group. During that time the major part of the research leading to theresults in Chapters 3 and 5 was conducted. After completing the work carried outat Rice University the research for the results in Chapter 4 was performed at theUniversity of Oulu in the second half of the year 1996.

    I wish to express my gratitude to my advisors Docent Jorma Lilleberg andProfessor Behnaam Aazhang for their comments and guidance throughout thework. My warmest thanks are also due to my supervisor Associate Professor PenttiLeppanen for providing me the opportunities to work on this interesting topic andfor guidance in my postgraduate studies. The help provided by Professor SavoGlisic, Professor Emeritus Juhani Oksman and Dr. Juha Ylitalo on various issuesand especially in getting the necessary financial support has also been invaluable. Iam grateful to the reviewers of the thesis Professor Timo Laakso from Helsinki Uni-versity of Technology, Espoo, Finland, and Professor Sergio Verdu from PrincetonUniversity, Princeton, New Jersey, USA.

    I would like to thank numerous members of the spread-spectrum research groupat the Telecommunication Laboratory of University of Oulu, as well as the facultyand students at Department of Electrical and Computer Engineering of Rice Uni-versity for the challenging environments to pursue research. The co-operation andjoint work with Matti Latva-aho has proven to be very fruitful. I am also gratefulto Markku Heikkila, who performed the simulations for Chapter 4. The discussionswith Dr. Kishore Kota and Dr. Markus Lang have been of great value. Thanks aredue to Jari Iinatti, Pertti Jarvensivu, Matti Latva-aho, Harri Saarnisaari, PekkaKaasila, and Tero Ojanpera who read and commented the manuscript. The help onvarious computer problems provided by Veikko Hovinen, Dr. Markus Lang, PekkaNissinaho, Dr. Jan Erik Odegard, Harri Saarnisaari, and Jari Sillanpaa has been

  • invaluable.The financial support provided by the Academy of Finland, Elektrobit, Ltd., the

    Graduate School in Electronics, Telecommunications, and Automation, Finnish AirForce, Nokia Mobile Phones, Ltd., Nokia Telecommunications, Ltd., Rice Univer-sity, the Technology Development Centre of Finland, the University of Oulu, aswell as the following Finnish foundations: Emil Aaltosen Saatio, Jenny ja AnttiWihurin Rahasto, Oy Nokia Ab:n Saatio, Tauno Tonningin Saatio, and TekniikanEdistamissaatio enabled this work and is thus gratefully acknowledged.

    I am grateful to my father Aarno and to my late mother Kaija for the help,support, and love they have provided for me throughout my life. The positiveattitude towards education in our home has been an important driving force formy later studies.

    I wish to express my deepest thanks to my family, my wife Hanna for the loveand support she has shown to me, and to my children Tuomas (three years) andKaisa (one year) for the understanding they have shown in their own ways. With-out Hannas highly positive attitude the thesis would not have been completed.

    Oulu, September 15, 1997 Markku Juntti

  • List of original publigations

    The thesis is in part based on the following original publications, which are referredin the text by Roman numerals:

    I Juntti M & Glisic S (1997) Advanced CDMA for wireless communications. In:Glisic SG & Leppanen PA (eds) Wireless Communications: TDMA VersusCDMA, Kluwer Academic Publishers, Chapter 4, p 447490.

    II Juntti MJ & Aazhang B (1995) Linear finite memory-length multiuser detec-tors. Proc. Communication Theory Mini-Conference (CTMC95) in conjunc-tion with IEEE Global Telecommunications Conference (GLOBECOM95),Singapore, November 1317, p 126130.

    III Juntti MJ, Aazhang B & Lilleberg JO (1996) Linear multiuser detectionfor R-CDMA. Proc. Communication Theory Mini-Conference (CTMC96)in conjunction with IEEE Global Telecommunications Conference (GLOBE-COM96) London, U.K., November 1822, p 127131.

    IV Juntti MJ & Aazhang B (1997) Finite memory-length linear multiuser de-tection for asynchronous CDMA communications. IEEE Transactions onCommunications 45(5): p 611622.

    V Juntti MJ (1997) Performance of decorrelating multiuser receiver with data-aided channel estimation. Proc. Communication Theory Mini-Conference(CTMC97) in conjunction with IEEE Global Telecommunications Confer-ence (GLOBECOM97), Phoenix, Arizona, USA, November 57.

    VI Juntti MJ, Latva-aho M & Heikkila M (1997) Performance Comparison ofPIC and Decorrelating Multiuser Receivers in Fading Channels. Proc. IEEEGlobal Telecommunications Conference (GLOBECOM97), Phoenix, Arizona,USA, November 38.

    VII Juntti MJ (1995) Linear multiuser detector update in synchronous dynamicCDMA systems. Proc. IEEE International Symposium on Personal, Indoorand Mobile Radio Communications (PIMRC95) Toronto, Ontario, Canada,September 2729, 3: p 980984.

  • VIII Juntti MJ, Aazhang B & Lilleberg JO (1996) Iterative implementation of lin-ear multiuser detection. Proceedings of Conference on Information Sciencesand Systems (CISS96), Princeton University, Princeton, New Jersey, USA,March 2022, 1: p 343348.

    IX Juntti MJ & Lilleberg JO (1996) Implementation aspects of linear mul-tiuser detectors in asynchronous CDMA systems. Proceedings of IEEE I

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